Every mail server and client is unique, and so are their error responses. When you send or receive email, errors may occur, and often the message you see contains the clue you need to fix it. Because we know it’s not always easy to decipher what they’re trying to tell you, we’ve collected a list of the most common email error messages and their variations.
This guide will help you with how to read those error messages, interpret them, and resolve them.
How Email Error Messages Work
Email servers communicate via protocols like SMTP, POP, and IMAP. When something goes wrong, they respond with status codes as 3-digit numbers that signal success, temporary issues, or permanent failures.
- Codes beginning with 2xx: success / no error
- Codes with 4xx: temporary failures (retry possible)
- Codes with 5xx: permanent errors needing action
TIP: Enhanced codes (e.g. 5.1.1, 5.2.1) give more detailed failure information, and you can find more detailed SMTP codes here.
How to Diagnose Your Error
- Locate the full error (bounce email or client dialog) and note the code and description.
- Determine if it’s a sending or receiving error.
- Match the code with the table below or an error in this guide.
- Cross-check your configuration settings (server names, ports, SSL/TLS, username/password).
- Try basic actions like resending again later (for 4xx codes).
Common SMTP / Bounce Codes and Their Meanings
| Code | What It Means | What to Try |
| 421 | Service not available (temporary) | Wait and then try and resend |
| 450 | Mailbox unavailable or server busy | Try later, verify that the mailbox exists |
| 451 | Local processing error | Retry again later; contact the recipient server if the issue persists |
| 452 | Insufficient system storage | The recipient needs to free space or try again later |
| 550 | Transaction failed/blocked | Verify the recipient address, check DNS / MX |
| 554 | Transaction failed / blocked | Mailbox not found/rejected |
Common Error Messages, What They Mean and How to Fix Them
Here’s some common error messages we’ve highlighted for you.
1. “550 Sender Verify Failed”
What This Means and How To Fix It
Full Mailbox
Mailbox storage quotas are set to 250MB by default, but some plans allow you to turn off the limit and set mailbox quotas to “unlimited.” If an email account is at or past its capacity, sending out from it won’t work, and you’ll get a “Sender Verify Failed” error.
Check the mailbox quota of an email account.
Mail Routing
Your domain’s email routing setting may be incorrect.
Check your MX records.
2. “Authentication Failed”
What This Means and How To Fix It
An “Authentication Failed” error means the email server cannot verify that your email access is authorised. This is typically due to a mistyped password, but it can also be caused by an incorrect username, connecting to the wrong server, or blacklisting.
Follow these steps to resolve the issue:
1. Reset your email password.
2. If you’ve recently changed where your domain points, or if you’ve upgraded or downgraded your hosting plan, wait up to 4 to 48 hours for your new DNS settings to propagate worldwide.
3. It’s also possible that your local IP is blacklisted. Wait 24 hours and try again, using Webmail in the meantime.
3. “Certificate Name Mismatch” or “Security Certificate Warning”
What This Means and How To Fix It
This error is a warning from your email application that the certificate for SSL that the server uses doesn’t match the name that your email application was expecting.
We’ve created a quick guide to help with troubleshooting a Security Warning Pop-Up in Outlook.
4. “IP Address Has Changed”
What This Means and How To Fix It
Every time you log into your Webmail account, your computer’s IP address is recorded. If your IP address changes while you’re logged in, Webmail won’t recognise your new connection as valid and will automatically log you out.
The easiest way to fix this error is to log in again so your new IP address is recorded.
However, if you’re seeing this error frequently, here are a few other things you can do:
- If possible, try using another internet connection, such as mobile data or Wi-Fi, and try again.
- If this resolves the issue, contact your Internet Service Provider (ISP) and ask to be given a static (unchanging) IP address so it won’t change while you’re logged in to Webmail.
- If that isn’t possible, ask your ISP if the “DHCP lease time” can be increased, which will allow your login sessions to be longer.
NOTE: Not all errors are failures. For example: 250 means “OK, message accepted”. More detailed codes can be found here.
TIP: Should you require more information regarding email errors, you can check out our Troubleshooting Guide.
Outlook and Client-Side Error Examples
Client-side errors mean something is wrong with the user’s browser, device, or local network, preventing the request from being processed correctly by the server, rather than a problem on the server itself. These are often indicated by 4xx HTTP status codes, such as a 404 Not Found error, and can be caused by issues like JavaScript errors, corrupted cache, or network connectivity problems.
- 0x8004210B is a timeout sending mail error in Outlook.
- 0x8004010f is a result of failures due to profile/mailbox access issues.
- “There was an error sending the message” is a generic send/receive failure in Outlook.
- In Webmail or a mobile app, you can commonly receive authentication errors (“login failed”), “server not found”, or SSL Certificate Warnings.
Quick Fixes and Troubleshooting
Invalid Recipient
- Correct the email address or remove any invalid addresses.
DNS / MX Record Issues
- Ensure your domain’s mail settings are correct.
Authentication Failures
- Reset the password, ensure “use same credentials” in the SMTP settings.
SSL / Certificate Mismatch
- Ensure your mail server’s certificate matches the hostname.
Firewall / Antivirus Interference
- Temporarily disable or adjust for the mail ports.
Retry After Delay
- For transient errors (4xx).
Persistent Rejects (5xx)
- Require changes before resending.
When The Error Isn’t Yours
Some errors are beyond your control (e.g. recipient server down, network issues). Some techniques, like greylisting, can result in 4xx temporary rejections even though your configuration is okay.
FAQs
I see the error “550 5.1.1 Mailbox not found.” What should I do?
This usually means the recipient address is invalid, or the mailbox was deleted. Double-check spelling, contact the recipient, or verify their server.
My Outlook shows “0x8004210B – send timeout.”
This happens when Outlook times out connecting to the SMTP server. Check your SMTP server, port, SSL settings, and network stability.
I got “421 Service not available.”
This is a temporary server issue. Wait and try sending again later.
Why did my email get “554 Transaction failed”?
This often means your message was blocked by spam filter rules or sender policy. Check content, sender IP reputation, or contact the recipient’s admin.
Do I always have to be technical to use this?
No, you can copy the error details and follow the steps above. If unsure, send the error message with screenshots, where possible, to Support, as it helps us diagnose faster.
Additional Resources
Top 10 Common Email Issues and How to Resolve Them
Resolving Email Error Messages When Sending to Gmail Accounts
Troubleshooting a Security Certificate Warning Pop-Up in Outlook
Setting Up Email in Outlook via Windows Control Panel
How to Set Up Email on Android for Your 1-grid Email Accounts
Settings to Configure Your 1-grid Email Accounts Across Devices
What Is Email Spoofing? How It Works and How to Protect Yourself
How to Spot a Scam Email
Email Headers FAQs
Email FAQs
How to Enable DKIM and SPF on Your Mail Domain
Why SSL? The Purpose of Using SSL Certificates
What is a Domain Name Server (DNS)?
How to Update Your Domain’s Nameservers at 1-grid
MailChannels FAQs
SpamTitan FAQs
Need Additional Support?
We’re Here to Help:
Understanding email error messages and figuring out how to fix them doesn’t have to feel so technical with this easy-to-reference guide. Stuck? Contact our Support Team for clarity and guidance (https://1grid.co.za/contact-us/). We’re ready to see how we can help!